Known optical diffusers have various surface profiles. For example, some diffusers comprise a plurality of microlenses that have a convex outline, as illustrated in FIGS. 1A to 1D (see C. Y. Wu, T. H. Chiang and C. C. Hsu, “Fabrication of microlens array diffuser films with controllable haze distribution by combination of breath figures and replica molding methods,” Optics Express, vol. 16, pp. 19978-19986 (2008); or S-I. Chang, J-B. Yoon, H. Kim, J-J. Kim, B-K. Lee and D. H. Shin, “Microlens array diffuser for a light-emitting diode backlight system,” Optics Letters, vol. 31, pp. 3016-3018 (2006); or T. R. M. Sales, S. Chakmakjian, G. M. Morris and D. J. Schertler, “Light Tamers,” Photonics Spectra, June 2004, Laurin Publishing.
A number of known optical diffusers are fabricated in glass, fused silica or plastic (polymer) materials as for example the diffusers shown in FIGS. 1A to 1C. These diffusers are intended for use at visible wavelengths (i.e., wavelengths of 0.45-0.65 μm). FIG. 1A shows a top-view photograph and a projection photograph of an array of partially spherical convex lenses having radii of curvature of about 3 μm and having a width, and center-to-center distance of approximately 6 μm. FIG. 1B shows a side-view photograph of an array of sugar-loaf shaped convex lenses having a height of about 10 μm and having a width and center-to-center distance of approximately the same dimension. FIG. 1C shows a side-view photograph of an array of conical convex lenses having a height of about 25 μm and having a width and center-to-center distance of approximately 10 μm. FIG. 1D shows a projection photograph of an array of partially spherical convex lenses having various radii of curvature larger than 100 μm and having center-to-center distances of 50-100 μm, with a lateral coherence length of those surface-feature variations being greater than 50 μm. Other known examples (not shown in the drawings) comprise engineered diffusers made by the company commercially known as RPC Photonics Incorporated for diffusing visible wavelength light (see G. M. Morris and T. R. M. Sales, “Structured screens for controlled spreading of light,” U.S. Pat. No. 7,033,736 B2 (2006)).
Other known diffusers comprise concave microlens arrays, as for example illustrated in FIGS. 2A-2C. FIG. 2A is a projection photograph of an array of partially spherical concave lenses having a radius of curvature larger than 25 μm and having center-to-center distances that are about twice that dimension. FIGS. 2B and 2C are top-view photographs of arrays of partially spherical concave lenses having a radius of curvature larger than 25 μm and having a center-to-center distance of 20-50 μm.
In order to produce a uniform spread of the beam of light, a surface-relief pattern consisting of lenses having a random variation or an engineered variation in the sag and in the spacing between adjacent lenses can be preferred to a pattern that has lens features with uniform shape and spacing (see also L. G. Shirley and N. George, “Diffuser radiation patterns over a large dynamic range. 1: Strong diffusers,” Applied Optics, vol. 27, pp. 1850-1861 (1988) or E. R. Mendez, et al., “Photofabrication of random achromatic optical diffusers for uniform illumination,” Applied Optics, vol. 40, pp. 1098-1108 (2001).)
Surface reliefs having other shapes than rounded convex or concave lenses also have been used for diffusers. For example, known diffusers are made from ground glass. FIG. 3A shows a projection photograph of a ground glass surface. The ground glass surface has prominent discontinuities and has features with high spatial frequency.
FIG. 3B shows a projection drawing of another known diffuser that comprises a pattern of cones or pyramids that have various heights and base widths. The heights of these cones are several times the wavelength of the light, and the base width of the cones as well as the variation in the vertical positions of the tips of the cones are at least 5-10 times the wavelength of the light.
The Inventors have noted that the known diffusers diffuse light over a wavelength range that is too broad for some applications, and that there exists a need for selective diffusers that efficiently diffuse traversing light having wavelengths within a band or range of shorter wavelengths but transmit with minimal effect light having wavelengths within a band or range of longer wavelengths.
As an example, FIG. 4 illustrates a dual mode optical seeker 10 wherein a wavelength selective mirror and lens 12 transmits light having a first wavelength to a diffuser 14 and a first sensor 16, and reflects light having a second wavelength to a lens 18 and a second sensor 20. Incoming light 22 from a scene 24 (not shown) that is illuminated partially by a laser source is reflected to the wavelength selective mirror 12 by a set of reflective optical elements 28. Because the diffuser 14 is not sufficiently wavelength selective, the wavelength selective mirror and lens 12 must separate out the second-wavelength light so that it is not coupled to the diffuser 14. Only the first-wavelength light is directed by wavelength selective mirror 12 to pass through diffuser 14 before reaching first sensor 16. A drawback of the structure illustrated in FIG. 4 is that, because it must place the first sensor 16 on the opposite side of wavelength-selective mirror 12 from the second sensor 20, part of the input aperture area is blocked. Thus, the overall diameter of the system must be larger in order to collect and project the desired amount of light onto the two sensors. The Inventors have determined that a selective diffuser that would efficiently diffuse traversing light having a shorter wavelength but would transmit light having a longer wavelength would allow making a more compact dual-mode optical seeker.